Variables and mutability

A variable is a name for a value. In Rust, variables are safe by default: they can't change unless you say so.

Let bindings

Use let to give a value a name. Then use the name to read the value.

1let age = 30;2println!("{age}"); // prints 30

You must create a variable before you use it.

Immutable by default

A plain let variable can't be changed. Trying to assign a new value is error E0384.

1let x = 5;2x = 6; // error E0384

This helps you: a value that never changes is easy to follow.

mut

Add mut when you want to change a variable.

1let mut score = 0;2score = 10;3println!("{score}"); // prints 10

mut lets you change the value, but not the type. A mut number stays a number.

Shadowing

A second let with the same name makes a brand new variable. The old one is hidden. The new one may even have a different type.

1let x = 5;2let x = x + 1;      // a new x, equal to 63let s = "   ";4let s = s.len();    // now s is a number: 3

Shadowing inside a { } block only lasts until the block ends.

Constants

A const is a value that never changes and is known when the program is built.

1const MAX_SCORE: u32 = 100;

• It always needs a type, like : u32.
• Its value can't come from a normal variable (error E0435).
• Its name is in CAPS by habit.
• You can never assign to it.

Remember

• let makes a variable; it can't change.
• let mut makes a variable you can change.
• E0384 means "you changed a variable that is not mut".
• A new let with the same name is shadowing, and it can change the type.
• const needs a type and a fixed value.